The present invention relates to equipment of the type used in surveying and construction and, more particularly, to a detector device for such applications which has an improved photodetector arrangement for detecting the position of reference light. The light may typically be a rotating laser beam that defines a reference plane of light or a stationary reference plane or cone of laser light. The reference plane may be horizontal or inclined, as dictated by the application. Additionally, a transmitter projecting the reference light may also regularly project a strobe light in synchronization with rotation of the beam so that the beam defines a specific azimuth angle.
Laser systems have commonly been employed in surveying and construction in which a laser beam is rotated in either a horizontal or a graded plane. U.S. Pat. No. 4,062,634, issued Dec. 13, 1977, to Rando, illustrates a laser transmitter which provides such a rotating reference beam. The rotating beam defines a plane, and various measurements can be made using the plane as a reference. For example, the elevation of a point remote from the laser transmitter may be measured using a rod on which a laser receiver is mounted. The bottom of the rod rests on the ground at the point where the measurement is to be made, and the operator moves the receiver along the rod to a position where it intercepts the laser beam, as indicated by a display on the receiver. One such laser receiver is shown in U.S. Pat. No. 4,240,208, issued Jun. 30, 1987, to Pehrson.
Additionally, laser systems have provided an indication of the azimuth angle of the laser beam relative to a reference direction (for example, true north) by transmitting a pulse of light, for example a strobe pulse, once during each rotation when the beam is directed in the reference direction. Such a system is shown, for example, in U.S. Pat. No. 6,643,004, issued Nov. 4, 2003, to Detweiler et al. Further, it is common for strobe lights to be used as warning lights at a construction site while machine control and surveying operations are taking place.
A laser receiver operating in such an environment must be able to distinguish between those times when the receiver is illuminated by the laser beam and those times when it is illuminated by a pulse of light from a strobe source. One way to distinguish between the two is to provide separate detector elements for the laser light and the strobe light, with optical filters to screen out all light except for light at the appropriate frequencies. This may not always be completely effective, however, and in addition to the unwanted interference, the optical filters may also attenuate light at the frequencies of interest, making such light harder to detect.
It is seen, therefore, that there is a need for a device and method for utilizing a plurality of photodetector arrays including a large number of photodetectors, and for accurately defining which one or ones of the photodetectors have been illuminated and by which light source the photodetectors have been illuminated by processing a number of output signals that have a continuously varying relationship.